Actin is a major cytoskeletal protein critical for cellular motility and mechanical strength, protein sorting and secretion, signal transduction and cell division. Since most of these functions are related to the properties of filamentous polymers of actin, F-actin, understanding of the dynamics of actin filament assembly and its structure is vital. The dynamic process of actin polymerization utilizes the ability of actin to hydrolyze ATP in its active site and to undergo nucleotide-dependent conformational transitions that influence filament dynamics. To date, details of F-actin structure and the mechanism of nucleotide-induced structural transitions in the filament are not yet known. The goal of this proposal is to elucidate details of the structure of F-actin at the atomic-resolution level. Furthermore, we would like to understand the mechanism of nucleotide-dependent structural changes within F-actin and how this mechanism controls binding interactions of actin with regulatory proteins. The actin filament network is associated with numerous diseases and abnormalities including the abnormal growth properties and metastatic potential of tumor cells. Therefore, results of this study would create the long anticipated structural background for developing new therapeutics targeting the actin cytoskeleton.